1. Field of the Invention
The invention relates to a voltage-controlled oscillator (VCO) used in a stereo demodulator.
2. Description of the Related Art
Progress has been made in recent years to provide adjustment-free initialization of the oscillation frequency for a VCO (voltage-controlled oscillator) that plays an important role in a PLL (phase-locked loop) operation of the demodulator of an FM (frequency modulation) tuner.
A conventional VCO system that requires no adjustment is shown in FIG. 3. The system is made up of an amplifier 11, a ceramic resonator 14, and a variable reactance circuit 13. The output of the amplifier 11 is positively fed back and is also connected to the ceramic resonator 14 and the variable reactance circuit 13, so that the output of the amplifier 11 serves as the output terminal of the VCO. FIG. 4 shows an equivalent circuit of the ceramic resonator 14. Assuming that Cv is the equivalent capacitance of the variable reactance circuit; Ca is the equivalent parallel capacitance of the ceramic resonator 14; Cr is the equivalent series capacitance; L is the equivalent series inductance; and r is the equivalent series resistance, then the output terminal 12 oscillates at a frequency obtained by equation (1): ##EQU1##
Thus, f.sub.osc (oscillation frequency) can be controlled by controlling the equivalent capacitance Cv of the variable reactance circuit. However, to initialize the VCO without adjustment, the initial oscillation frequency (with the variable reactance circuit uncontrolled) must be a parallel resonance frequency inherent in the ceramic resonator 14. For this reason, the capacitance component Cv when the variable reactance circuit 13 is not controlled is zero, while the capacitance component must be variable from a negative value to a positive value when the variable reactance circuit 13 is driven into control.
The output terminal 12 of the above-mentioned conventional system, i.e., the load at the output of the amplifier 11, consists of the variable reactance circuit 13 and the ceramic resonator 14, and a synthetic load Z is thus given by equation (2): ##EQU2## where Cx=Ca+Cv
If the system is in resonance, then .omega. is given by equation (3): ##EQU3##
From equations (2), (3), the absolute value .vertline.Z.vertline. of the load at the output terminal 12 can be obtained as follows: ##EQU4##
It is understood from equations (1) and (4) that the oscillation level increases with increasing resonance frequency.
Since the oscillation level at the output terminal 12 is restricted by the input/output dynamics of the amplifier 11 and the variable reactance circuit 13, oscillation over such input/output dynamics is beyond control. This means that the high frequency side of desired variable oscillation frequencies cannot be varied for the VCO, and to select the desired variable frequency range, the oscillation level must be set to a small value. In other words, the amplifier 11 be designed so as to have a small output current capacity. This impairs the oscillation capacity of the VCO and thereby makes it difficult to provide a stable supply of oscillations. Further, since the equivalent capacitance Cv of the variable reactance circuit 13 changes at an equal value positively and negatively from equation (1), the variable frequency range on the high frequency side becomes wider, making the capture range of the PLL (phase-locked loop) asymmetric.